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Vintage AP-70 resurrected - how to replace the contact strips


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Posted (edited)

Hi everyone, 

 

I have just successfully resurrected my vintage Casio Celviano AP-70 digital piano. This 30 year old piano has three main weak points that wear over time:

 

  • Hammer rubber pad
  • Hammer plastic
  • Silicone contact pad strips

 

The rubber pad receives the pressure from the actual key with the key sliding over the rubber like a saw. And while the contact parts are lubricated with silicone grease, like a good saw they cut through the rubber over time. I’m certain the age also plays a part in the rubber becoming brittle. Either way, when the rubber is gone, the key rides on the hammer plastic, which causes the key to drop 1mm, which is annoying and hard to fix. I solved this by swapping these hammers from the center to the lesser used high and low ends. 
 

Number two, the hammer plastic tends to crack, making the hammers brittle. You can remove the hammer through the top, no need to remove the whole sheet metal support. Repair using epoxy glue is possible, but care must be taken that the hammers still fit through the metal support. 
 

The main failure part are the silicone contact strips. Casio used to carrry replacement mini-strips of 3 contacts, but these have been long discontinued. When the silicone fails after being hit by the hammers millions of times, tears develop or the little contact towers bend, causing dead, loud, delayed or otherwise malfunctioning keys. 

I have researched that AP-420 contact strips are available on AliExpress and close enough to the AP-70 strips to work! I had to cut off three on the orientation „knobs“, but the others fit into the existing holes.

 

Orientation is important, the impact part of the pad is angled and must be parallel to the hammer. If installed incorrectly, all keys sound faint. With the correct orientation however, the two contact towers are reversed compared to the original contacts. The piano measures the time between the two contacts closing and then determines which volume to play. Obviously, if they close in the wrong order, this doesn’t work, and the piano stays silent,

 

Now, these contacts are called „first contact“ (FC) and „second contact“ (SC), so they must be reversed in the wiring. Fortunately this is quite simple. After disassembling the keyboard and removing about 2648 screws, the three circuit boards can be removed. The silicone strips are under the circuit boards. The two connectors are color coded and don’t have a lock. Try not to bend the flex ribbons too often or they might break.

 

Now, the keys are arranged in groups of 8 keys. There are 11 groups = 88 keys. Each group of 8 shares a common SC and FC wire trace. Since the circuit board is single sided, most of these can be traced to two side by side wire bridges. Cut the wires and rewire them as an „X“ using isolated wire. Two sets of of them are more difficult though, so the traces must be cut, scraped and rewired with thin isolated wire.

 

After this, reinstall the PCBs, reconnect the connectors and fasten 3 screws on each circuit board. The check each hammer carefully for correct operation. If everything works, reinstall all screws and then all keys. Recheck proper operation, then close the piano again and have a beer - you deserve it. 

Edited by Fieroluke
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